1. Field of the Invention
The present invention relates to a heat treatment susceptor for holding a substrate including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like which is to be heat-treated by being exposed to light during the heat treatment thereof, and a heat treatment apparatus provided with the heat treatment susceptor.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in the semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, for example, about 1000° C. to about 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by the use of the above-mentioned lamp annealer which raises the temperature of the semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the semiconductor wafer are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to an apprehension about a hindrance to good device formation.
To solve the problem, there has been proposed a technique for exposing the surface of a semiconductor wafer to a flash of light by using a xenon flash lamp and the like to raise the temperature of only the surface of the semiconductor wafer implanted with ions in an extremely short time (several milliseconds or less). The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and approximately coincides with a basic absorption band of a silicon semiconductor wafer. It is therefore possible to rapidly raise the temperature of the semiconductor wafer, with a small amount of light transmitted through the semiconductor wafer, when the semiconductor wafer is exposed to a flash of light emitted from the xenon flash lamp. Also, it has turned out that a flash of light emitted in an extremely short time of several milliseconds or less can achieve a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deeply diffusing the ions.
In a heat treatment apparatus employing such xenon flash lamps, the area in which the plurality of xenon flash lamps are arranged is much greater than the area of the semiconductor wafer. Nevertheless, the illuminance in a peripheral portion of the semiconductor wafer is somewhat lower than that in an inner portion thereof. In particular, a wafer having a diameter as large as 300 mm exhibits a large degree of lowering in illuminance in the peripheral portion thereof to result in the poor uniformity of a within-wafer illuminance distribution.
To solve such a problem, U.S. Pat. No. 6,856,762 discloses a heat treatment apparatus in which a geometrical pattern of ground glass is formed in a region of a diffuser provided between xenon flash lamps and a semiconductor wafer, the region being positioned over a portion (or inner portion) other than a peripheral portion of the semiconductor wafer. Thus, this heat treatment apparatus decreases the light transmittance of the region to decrease the illuminance in the inner portion of the semiconductor wafer during flash heating, consequently providing a uniform within-wafer illuminance distribution.
However, it has been found that a heat treatment apparatus employing a xenon flash lamp presents not only the non-uniformity of a temperature distribution in a radial direction of the semiconductor wafer but also the non-uniformity of a temperature distribution in a circumferential direction of the semiconductor wafer having the same radius. Specifically, there have been cases in which low temperature regions referred to as cold spots are formed only in part of the peripheral portion of the semiconductor wafer. For elimination of such non-uniformity of the temperature distributions, it is impossible to adjust the light source of the xenon flash lamp. It is also difficult for a hot plate for preheating the semiconductor wafer prior to the flash heating to eliminate the non-uniformity of the temperature distribution in the circumferential direction of the semiconductor wafer having the same radius.